1. Field of the Invention
The present invention generally relates to titanium aluminide, cast (or mechanical part) made from the titanium aluminide, and method of making the cast, and more particularly relates to those used in manufacture of mechanical parts of a turbocharger mounted on a diesel engine operating under an elevated temperature for a long period.
2. Description of the Related Art
Titanium aluminide is an alloy of Al and Ti. Because of its characteristics such as lightweight and high strength, TiAl is commonly used in rotating parts of jet engines and automobile engines. When TiAl is used in mechanical parts of a vehicle such as parts of a turbocharger of a diesel engine, which are subjected to a very high temperature for a considerable time of period, however, additional considerations and improvements are needed in terms of mass productivity, cost effectiveness, creep resistance, oxidation resistance, etc. Specifically, mechanical parts made from conventional TiAl are mostly fabricated by forging, but the forging process is not suited for mass production. Since automobiles are made in a large number, it is not practical to manufacture the parts of the turbocharger by the forging process.
In the meantime, it is known that the creep resistance can be improved by adding third and/or fourth element such as W, Ta, Nb and Cr. However, addition of the third/fourth element would greatly degrade precision castability. The mechanical parts of the engine should often be made by precision casting. It is also known that the creep resistance can be raised by forging if the forging is performed in a manner to control the structure. However, this requires complicated heat treatment, which in turn results in raised cost.
Further, the conventional TiAl is poor in oxidation resistance under high temperature. Specifically, the surface of the product is oxidized if the surrounding temperature exceeds 700° C., and the resulting scale peels off. Accordingly, the product made from the conventional TiAl cannot be used for the turbocharger or the like that is designed to operate in an environment over 700° C.